Seals for automotive body applications (such as, windows, hoods, trunks and doors) and building applications (such as window glazing gaskets and weather strips) should be dimensional stable, provide low compression set, and offer outstanding sealing characteristics over a broad temperature range. Such seals must be capable of sealing out noise, wind and water, while providing long-term ultraviolet light resistance. At the same time the material used in the seal must offer the degree of flexibility required for the particular application. Window and door weather stripping for automobiles and trucks is a high volume application for such seals. However, seals offering essentially the same characteristics are also needed for sun roof seals, handle gaskets, window spacers, window guides, lock seals, windshield wiper pivot seals and in building applications such as window glazing gaskets and weather seals.
Rubbery blends of polyvinyl chloride (PVC) with a nitrile rubber (NBR) have sometimes been used in seals for automotive body applications. The nitrile rubber is included in such blends as a permanent modifier for the PVC that provides it with a higher degree of flexibility. However, the utilization of standard nitrile rubber in such blends typically results in only moderate compression set characteristics. It is very important for seals to have good compression set characteristics in most applications. For instance, improved resistance to water leaks and wind noise can be attained by utilizing a seal which has low compression set characteristics.
It is known from the teachings of UK Patent Application No. 9214969.9 that low compression set characteristics can be improved by utilizing a technique known as xe2x80x9cdynamic vulcanizationxe2x80x9d via free radical generators, such as azo compounds or organic peroxides. However, this xe2x80x9cdynamic vulcanizationxe2x80x9d technique suffers from the weakness that the azo compounds or organic peroxides required reduce the thermal stability of the polyvinylchloride resin and the ultraviolet light resistance of the nitrile rubber. There is also an increased risk of early crosslinking during processing which leads to scorching and reduced recyclability.
U.S. Pat. No. 5,552,468 discloses a highly crosslinked nitrile rubber composition which can be blended with polyvinyl chloride to make compositions having excellent characteristics for seals and gaskets including dimensional stability, low compression set, and outstanding sealing characteristics, said highly crosslinked nitrile rubber composition being comprised of (1) a highly crosslinked nitrile rubber having repeat units which are derived from (a) 1,3-butadiene, (b) acrylonitrile, and (c) a crosslinking agent, wherein said highly crosslinked nitrile rubber has a Mooney viscosity of about 50 to about 120, a swelling index of less than about 10 percent, a mill shrinkage of less than 10 percent, and a gel content of greater than 90 percent; and (2) from about 1 to about 30 phr of a plasticizer.
Today there is a growing demand for seals and gaskets that do not contain halogens, such as polyvinyl chloride. This is because industry in general and the automotive industry in particular are attempting to reduce the amount of halogenated materials in industrial products for improved recyclability and other environmental reasons.
U.S. patent application Ser. No. 09/769,636, filed on Jan. 25, 20001, pending discloses a polymeric composition comprising (1) a compatibilizing polymer selected from the group consisting of ethylene vinyl acetate and ethylene alkyl acrylates, (2) a polyolefin, (3) a free radical generator, and (4) a rubbery polymer which is comprised of repeat units which are comprised of (a) butyl acrylate, or optionally a mixture of butyl acrylate and 2-ethylhexyl acrylate containing up to about 40 percent 2-ethylhexyl acrylate, (b) at least one member selected from the group consisting of methyl methacrylate, ethyl methacrylate, methyl acrylate and ethyl acrylate, (c) acrylonitrile, (d) styrene and (e) a crosslinking agent. However, these polymeric compositions do not have enough resistance to compression set to be useful in most applications as seals and gaskets.
This invention relates to halogen-free polymeric compositions that can be employed in seals and gaskets for automotive and building application. These polymeric compositions have good tensile strength, tear resistance, ultra violet light resistance, and resistance to compression set. More specifically, this invention discloses a polymeric composition that has excellent characteristics for use in manufacturing seals and gaskets, said polymeric composition comprising (1) a compatibilizing polymer selected from the group consisting of ethylene vinyl acetate and ethylene alkyl acrylate co(polymer), (2) a polyolefin, (3) a curative system, (4) a rubbery polyolefin, and (5) a rubbery polymer which is comprised of repeat units which are comprised of (a) butyl acrylate, or optionally a mixture of butyl acrylate and 2-ethylhexyl acrylate containing up to about 40 percent 2-ethylhexyl acrylate, (b) at least one member selected from the group consisting of methyl methacrylate, ethyl methacrylate, methyl acrylate and ethyl acrylate, (c) acrylonitrile, (d) styrene and (e) a crosslinking agent. The polymeric compositions of this invention can also optionally contain a plasticizer. The rubbery polyolefin can be an ethylene-propylene-diene monomer polymer (EPDM), and the curative system will typically be a free radical generator.
The rubbery polymers that can be utilized in making the blends of this invention are synthesized utilizing a free radical emulsion polymerization technique. These rubbery polymers are comprised of repeat units which are derived from (a) butyl acrylate, or optionally a mixture of butyl acrylate and 2-ethylhexyl acrylate containing up to about 40 percent 2-ethylhexyl acrylate, (b) methyl methacrylate, ethyl methacrylate, methyl acrylate or ethyl acrylate, (c) acrylonitrile, (d) styrene and (e) a crosslinking agent. The crosslinking agent is typically a multi-functional acrylate, a multi-functional methacrylate or divinylbenzene. Some specific examples of crosslinking agents which can be used include ethylene glycol methacrylate, divinylbenzene and 1,4-butanediol dimethacrylate.
Technically, the rubbery polymers of this invention contain repeat units (chain linkages) which are derived from (a) butyl acrylate, or optionally a mixture of butyl acrylate and 2-ethylhexyl acrylate containing up to about 40 percent 2-ethylhexyl acrylate, (b) methyl methacrylate, ethyl methacrylate, methyl acrylate or ethyl acrylate, (c) acrylonitrile, (d) styrene and (e) a crosslinking agent. These repeat units differ from the monomers that they were derived from in that they contain one less carbon-carbon double bond than is present in the respective monomer. In other words, a carbon-to-carbon double bond is consumed during the polymerization of the monomer into a repeat unit in the rubbery polymer. Thus, in saying that the rubbery polymer contains various monomers, in actuality means that it contains repeat units which are derived from those monomers.
The rubbery polymers of this invention will normally contain (a) from about 40 weight percent to about 80 weight percent butyl acrylate, or optionally a mixture of butyl acrylate and 2-ethylhexyl acrylate containing up to 40 weight percent 2-ethylhexyl acrylate, (b) from about 5 weight percent to about 35 weight percent methyl methacrylate, ethyl methacrylate, methyl acrylate or ethyl acrylate, (c) from about 4 weight percent to about 30 weight percent acrylonitrile, (d) from about 3 weight percent to about 25 weight percent styrene and (e) from about 0.25 weight percent to about 8 weight percent of a crosslinking agent.
Such rubbery polymers will preferably contain (a) from about 50 weight percent to about 80 weight percent butyl acrylate, or optionally a mixture of butyl acrylate and 2-ethylhexyl acrylate containing up to about 40 percent 2-ethylhexyl acrylate, (b) from about 3 weight percent to about 25 weight percent of at least one member selected from the group consisting of methyl methacrylate, ethyl methacrylate, methyl acrylate and ethyl acrylate, (c) from about 6 weight percent to about 30 weight percent acrylonitrile, (d) from about 5 weight percent to about 18 weight percent styrene and (e) from about 0.5 weight percent to about 4 weight percent of a crosslinking agent.
The rubbery polymer will more preferably be comprised of repeat units which are derived (a) from about 55 weight percent to about 75 weight percent butyl acrylate, or optionally a mixture of butyl acrylate and 2-ethylhexyl acrylate containing up to about 40 percent 2-ethylhexyl acrylate, (b) from about 5 weight percent to about 20 weight percent of at least one member selected from the group consisting of methyl methacrylate, ethyl methacrylate, methyl acrylate and ethyl acrylate, (c) from about 10 weight percent to about 25 weight percent acrylonitrile, (d) from about 8 weight percent to about 14 weight percent styrene and (e) from about 1 weight percent to about 3 weight percent of a crosslinking agent. The percentages reported in this paragraph are based upon the total weight of the rubbery polymer.
U.S. Pat. No. 5,616,651 describes a process for synthesizing rubbery polymers that can be employed in the blends of this invention in detail. The teachings of U.S. Pat. No. 5,616,651 are accordingly incorporated herein by reference in their entirety. Rubbery polymers that can be used in the practice of this invention are also sold by The Goodyear Tire and Rubber Company as Sunigum(copyright) rubber.
Some representative examples of ethylene alkylacrylate polymers that can be used as compatibilizing polymers in the blends of this invention include ethylene/methyl acrylate copolymers, ethylene/ethyl acrylate copolymers, ethylene/propyl acrylate copolymers, ethylene/butyl acrylate copolymers, ethylene/butyl acrylate/carbon monoxide terpolymers, ethylene/ethyl acrylate/maleic anhydride terpolymers and ethylene/acrylate/glycidyl methacrylate terpolymers. The ethylene-vinyl acetate (EVA) copolymers that can be used as compatibilizing polymers in the practice of this invention will typically contain from about 7 weight percent to 40 weight percent of the vinyl acetate monomer. Such ethylene-vinyl acetate copolymers are commercially available from a wide variety of sources. For instance, Du Pont sells ethylene-vinyl acetate copolymers under the trade name Elvax(copyright) and Elf-Atochem sells ethylene-vinyl acetate copolymers under the trade name Evatane(copyright).
A wide variety of polyolefins can be utilized in the blends of this invention. Some representative examples of polyolefins that can be used include polyethylene, polyethylene synthesized with metallocene catalyst systems, polypropylene, polyethylene/polypropylene copolymers and other thermoplastic polyolefins. It is preferable for the polyolefin to be semi-crystalline.
Conventional rubbery polyolefins can be utilized in the blends of this invention. Some representative examples of rubbery polyolefins that can be used include ethylene-propylene rubber (EPM), ethylene-propylene-diene monomer rubber (EPDM), polyethylene synthesized with metallocene catalyst systems, butyl rubber, styrene-ethylene butylene-styrene rubber (SEBS), and ethylene-acrylic rubber. Such rubbery polyolefins are commercially available from a wide variety of sources.
Different curative systems, such as sulfur, free radical generators, phenolic resins, quinone derivatives, maleimides, and mixtures of these systems, can be used in the blends of this invention. However, phenolic resins are generally preferred.
The free radical generator can be virtually any chemical compound that will produce free radicals when heated to a temperature of at least about 90xc2x0 C. The free radical generator will preferably produce free radicals after being heated to a temperature of at least about 100xc2x0 C. The free radical generators that can be utilized include 2,2xe2x80x2-azo-bis(2-acetoxypropane), 2,5-dimethyl-2,5-bis(tert-butyl-peroxy)-hexyne, 2,5-dimethyl-2,5-bis(tert-butyl-peroxy)-hexane, dibutyl peroxide, tert-butylcumyl peroxide, bis(2-tert-butylperoxypropyl-(2))-benzene, dicumyl peroxide, bis(tert-butyl)-3,3 ethylene butyrate, bis(tert-butyl) 4,4-butyl valerate, tert-butyl peroxy-3,5,5 trimethylhexanoate, tert-butyl perbenzoate, and bis(tert-butyl peroxy) 1,1-trimethyl-3,3,5-cyclohexane, and the like. It is normally preferred to use a phenolic resin as the free radical generator because improved resistance to compression set results.
In cases where a free radical generator is used as the curative system it is preferred for a cocuring agent, such as 1,1xe2x80x2-(methylene-4,1-phenylene) bis-maleimide, m-phenylene-diamine-bis maleimide, triallyl cyanurate, triallyl isocyanurate, diallyl terephthalate, diallyl fumarate, triallyl mellitate, triallyl citrate, triallyl phosphate, trimethylol propane trimethacrylate, diallyl phthalate, ethylene-glycol-dimethacrylate, zinc di(meth)acrylate, magnesium di(meth)acrylate, m-phenylene-diamine-bis-maleic-imid, sulfur, or the like, to be included in the blend to reduce the mixing time required to crosslink the rubbery polymer. For a list of suitable cocuring agents, see T H Kempermann, Rubber Chem and Tech, July/August 1988, Volume 61, No 3.
A wide variety of plasticizers which are compatible with the polyolefin resin and/or the rubbery polymer can also optionally be employed. The plasticizers employed will normally be in the liquid state and can be polar or non-polar in nature. The term xe2x80x9cpolar plasticizerxe2x80x9d as used herein means a plasticizer that is compatible with a polar polymer, such as polyvinylchloride (PVC). Some representative examples of polar plasticizers include phosphate derivative plasticizers, phthalate derivative plasticizers, adipate derivative plasticizers, azelate derivative plasticizers, tallate derivative plasticizers, sebacate derivative plasticizers, and trimellitate derivative plasticizers. The term xe2x80x9cnon-polarxe2x80x9d as used herein means a plasticizer that is compatible with a non-polar polymer, such as polyolefins and the like. Some representative examples of non-polar plasticizers include conventional hydrocarbon oil diluents or process oils, such as paraffin oil, naphthalene oil, and the like.
The polymeric compositions described herein can be made by initially blending the compatibilizing polymer, the curing system, the rubbery polymer and the rubbery polyolefins in a first step. This step can be conducted at a temperature that is either below or above the temperature at which the free radical generator produces free radicals. For instance, it can be carried out at a temperature below the decomposition temperature of the peroxide that was used as the free radical generator. The desired polyolefin, the rubbery polyolefin, and optionally the plasticizer can subsequently be blended into the blend of the compatibilizing polymer, the curative system and the rubbery polymer in a second mixing step. This step must be conducted at a temperature above the temperature at which the dynamic vulcanization is carried out in the event that the first step is conducted at a lower temperature. The five or six components of the blend can, of course, also be mixed together in a single mixing step that is carried out above the temperature at which the free radical generator produces free radicals (above the temperature at which dynamic vulcanization takes place).
The blends of this invention will typically contain from about 1 part to about 500 parts of the polyolefin polymer, from about 1 parts to about 500 parts of the rubbery polyolefin, optionally, from about 1 part to about 400 parts of the plasticizer, from about 1 part to about 200 parts by weight of the compatibilizing polymer, about 0.05 parts to about 10 parts of the curative system, and optionally, about 0.01 parts to about 10 parts of the cocuring agent per 100 parts by weight of the rubbery polymer. It is typically preferred for the polymeric compositions of this invention to contain from about 5 parts to about 300 parts of the polyolefin polymer, from about 5 parts to about 300 parts of the rubbery polyolefin, optionally, from about 5 parts to about 300 parts of the plasticizer, from about 5 to about 100 parts by weight of the compatibilizing polymer, about 0.05 parts to about 6 parts of the curative system, and optionally, about 0.05 parts to about 6 parts of the cocuring agent per 100 parts by weight of the rubbery polymer. It is typically most preferred for the polymeric compositions of this invention to contain from about 10 parts to about 200 parts of the polyolefin polymer, from about 10 parts to about 200 parts of the rubbery polyolefin, optionally, from about 10 parts to about 200 parts of the plasticizer, from about 10 to about 50 parts by weight of the compatibilizing polymer, about 0.1 parts to about 4 parts of the curative system, and optionally, about 0.1 parts to about 4 parts of the cocuring agent per 100 parts by weight of the rubbery polymer.
Other ingredients, such as antioxidants and fillers, can be added to the polymeric composition to attain the desired properties and to reduce cost. Various colorants and/or pigments will typically also be added to the composition to attain the desired color.